Stainless Steel Coil Tube (Coiltube) for Solar Thermal Systems — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically designed for integration into solar thermal energy systems, particularly plastic coil tube solar water heaters. These coil tubes serve as the critical heat exchange medium, transferring thermal energy from the solar collector to the water storage tank with maximum efficiency and minimal heat loss.
The product is engineered to withstand prolonged exposure to thermal cycling, UV radiation, and chemically aggressive water environments, ensuring a service life exceeding 15–20 years under normal operating conditions. The coiled configuration maximizes the heat transfer surface area within a compact volumetric footprint, making it an optimal choice for both residential and commercial solar water heating installations.
2. Material & Properties
2.1 Base Material Composition
The coil tubes are fabricated from high-grade austenitic stainless steel, primarily conforming to the following grades:
- 304 (UNS S30408 / AISI 304) — General-purpose grade offering excellent corrosion resistance and formability, suitable for standard solar water heater applications.
- 316L (UNS S31603 / AISI 316L) — Enhanced grade with molybdenum (Mo) addition, providing superior resistance to chloride-induced pitting and crevice corrosion, ideal for coastal or industrial environments.
- 321 (UNS S32100 / AISI 321) — Stabilized grade with titanium (Ti) addition, resistant to intergranular corrosion at elevated temperatures.
2.2 Key Physical and Mechanical Properties
| Property | Specification |
|---|---|
| Density (密度) | 7.93 g/cm³ |
| Thermal Conductivity (热导率) | 16.3 W/(m·K) at 20°C; 24.9 W/(m·K) at 100°C |
| Specific Heat Capacity (比热容) | 500 J/(kg·K) |
| Coefficient of Thermal Expansion (热膨胀系数) | 17.3 × 10⁻⁶ /°C (20–100°C) |
| Tensile Strength (抗拉强度) | ≥ 520 MPa (ASTM A270) |
| Yield Strength (屈服强度) | ≥ 205 MPa |
| Elongation (延伸率) | ≥ 40% |
| Operating Temperature Range (工作温度范围) | −196°C to +450°C |
| Corrosion Resistance (耐腐蚀性) | Excellent resistance to atmospheric, freshwater, and dilute acid environments |
3. Specifications
| Parameter | Specification / Range |
|---|---|
| Outer Diameter (外径, OD) | 12 mm, 16 mm, 20 mm, 25 mm, 32 mm (customizable) |
| Wall Thickness (壁厚, WT) | 0.5 mm – 2.0 mm (standard: 0.8 mm – 1.2 mm) |
| Coil Diameter (盘管直径) | 50 mm – 300 mm (configurable based on system design) |
| Coil Length (盘管展开长度) | 3 m – 50 m (custom lengths available) |
| Tube Pitch (管间距) | 5 mm – 20 mm (optimized for laminar/turbulent flow) |
| Surface Finish (表面光洁度) | Bright annealed (BA) or hairline finish; Ra ≤ 0.8 μm |
| Pressure Rating (压力等级) | 0.6 MPa – 1.6 MPa (working pressure) |
| End Connection (端部连接) | Flared, threaded (G1/2", G3/4"), grooved, or welding ends |
| Applicable Standards (适用标准) | ASTM A269, ASTM A270, GB/T 12771, GB/T 14976 |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection (原材料选择): Cold-rolled stainless steel strip conforming to ASTM A240 or equivalent standards, procured from certified mills with full traceability.
- Coil Winding (盘绕成型): Precision CNC-controlled winding machines form straight tube stock into uniform coil configurations with controlled pitch and diameter tolerances (±0.5 mm).
- Seam Welding (缝焊): High-frequency induction or TIG (Tungsten Inert Gas) welding ensures hermetic seams with penetration strength ≥ 80% of base metal.
- Heat Treatment (热处理): Solution annealing at 1050–1150°C followed by rapid water quenching to achieve a fully austenitic microstructure, eliminating residual stresses and optimizing corrosion resistance.
- Pickling & Passivation (酸洗钝化): Immersion in nitric acid or citric acid solution to remove oxide scale, followed by passivation treatment to form a stable chromium oxide protective layer (Cr₂O₃).
- Dimensional Inspection (尺寸检测): Automated optical and ultrasonic testing for wall thickness uniformity, coil geometry verification, and weld integrity assessment.
- Pressure Testing (压力测试): Hydrostatic pressure testing at 1.5× the rated working pressure, held for a minimum of 30 seconds to verify leak-free performance.
- Packaging (包装): Protective wrapping with moisture barriers and shock-absorbing materials to prevent surface damage during transportation.
4.2 Quality Control Measures
- Spectroscopic Analysis (光谱分析): PMI (Positive Material Identification) verification of chemical composition using optical emission spectrometry (OES).
- Ultrasonic Testing (超声波检测, UT): Full-length UT scanning for internal defects including laminations, inclusions, and incomplete weld fusion.
- Visual Inspection (目视检测): 100% visual examination for surface imperfections, discoloration, and geometric deviations.
- Hardness Testing (硬度测试): Rockwell or Vickers hardness verification to confirm proper annealing treatment (HV ≤ 250 for annealed condition).
5. Applications
5.1 Primary Applications
- Residential Solar Water Heaters (家用太阳能热水器): The coil tube functions as the immersion heat exchanger within the storage tank, enabling indirect water heating through efficient thermal conduction from the solar collector loop.
- Commercial Solar Thermal Systems (商用太阳能热利用系统): Large-diameter coil assemblies serve as the primary heat exchange element in multi-thousand-liter storage tanks for hotels, hospitals, and industrial facilities.
- Industrial Process Heating (工业过程加热): Integration into closed-loop solar preheating systems for chemical, food processing, and textile manufacturing applications.
- Heat Pump Systems (热泵系统): Application as an evaporator or condenser coil in air-source and ground-source heat pump units.
- Geothermal Energy Systems (地热能系统): Utilization in closed-loop geothermal heat exchange configurations.
5.2 System Integration Advantages
The coil tube configuration eliminates the need for direct contact between the heat transfer fluid and the stored water, thereby preventing water quality degradation, scale formation, and potential contamination. This indirect heating approach extends the service life of both the storage tank and the solar collector components.
6. Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent manufacturing quality |
| ISO 14001:2015 | Environmental Management System certification for sustainable production practices |
| CE Marking | Conformity with European health, safety, and environmental protection standards |
| Solar Keymark (EN 12975) | European certification for solar thermal components and systems |
| ASME B31.3 | Process piping standards compliance for pressure-containing applications |
| GB/T 12771 | Chinese national standard for welded austenitic stainless steel tubes for general application |
| NORSOK M-650 | Material certification for offshore and marine environments (where applicable) |
7. Unique Selling Points
- Superior Corrosion Resistance: The austenitic stainless steel construction provides outstanding resistance to water-induced corrosion, eliminating the need for sacrificial anodes or cathodic protection systems.
- High Thermal Efficiency: The optimized coil geometry and high thermal conductivity of stainless steel achieve thermal transfer efficiencies exceeding 92% under standard operating conditions.
- Exceptional Durability: Resistance to thermal fatigue and mechanical stress ensures reliable operation across 20,000+ thermal cycling events without performance degradation.
- Hygienic Surface Quality: The bright annealed finish with Ra ≤ 0.8 μm prevents biofilm accumulation, making the product suitable for potable water applications.
- Full Traceability: Each coil tube batch is accompanied by a mill test certificate (MTC) per EN 10204 Type 3.1, enabling complete material traceability from raw stock to finished product.
- Customization Capability: Flexible manufacturing parameters allow adaptation to specific system designs, including custom coil diameters, tube lengths, pitch configurations, and end connection types.
- Environmental Sustainability: Stainless steel is 100% recyclable without degradation of material properties, supporting circular economy principles and reducing lifecycle environmental impact.
8. Installation & Maintenance Guidelines
Installation: Ensure proper support brackets at intervals not exceeding 500 mm to prevent tube sagging. Maintain minimum clearance of 25 mm between the coil assembly and the tank wall to facilitate natural convection circulation. Use compatible gaskets and sealants rated for continuous temperatures up to 150°C.
Maintenance: Annual inspection of weld integrity and surface condition is recommended. In hard water environments (CaCO₃ > 150 mg/L), periodic descaling with citric acid solution (5–10% concentration) maintains optimal heat transfer performance. Avoid contact with abrasive cleaning agents or tools that may compromise the passive oxide layer.
9. Conclusion
Wenzhou Jixing Metal Materials Co., Ltd.'s Stainless Steel Coil Tube represents a benchmark in solar thermal heat exchange technology, combining metallurgical excellence with precision manufacturing to deliver reliable, efficient, and long-lasting performance. The product's adherence to international standards and comprehensive quality assurance framework makes it an ideal choice for engineers and system integrators seeking dependable solutions for solar energy utilization across diverse application sectors.